Use secrecy to store sensible data

[robin-nitrokey]

Once iqlusioninc/crates#1094 is fixed, we should consider using the secrecy crate to store pins and hashes.

[sosthene-nitrokey]

Trussed should probably zeroize the interchange too for that to be truly useful.

[sosthene-nitrokey]

We should also consider ways to securely erase data stored persistently, but I'm under the impression that littlefs does not offer such a feature.